1. Field of the Invention
The present invention relates to a reflective display apparatus using a polarizing beam splitter (PBS), and more particularly, to a reflective display apparatus enhancing a contrast ratio by controlling a switching angle with respect to each molecular axis of a compensator and ferroelectric liquid crystal display device.
2. Description of the Related Art
A contrast ratio is a scale representing how definitely an image is viewed. An image is viewed better as a difference in luminance is greater. The contrast ratio is a value which is obtained by dividing a luminance value of the white state by that of the black state at the center of a panel. The luminance of the black state has a value smaller than that of the white state. Thus, it can be seen that the contrast ratio is influenced more by the luminance of the black state. Finally, the contrast ratio becomes higher as the luminance value of the black state is smaller.
FIG. 1 is a schematic diagram showing a configuration of a conventional reflective display apparatus using a PBS.
In FIG. 1, a P polarized beam of a beam incident to a polarizing beam splitter (PBS) 11 transmits through the PBS 11 and an S polarized beam reflects from the PBS 11. The beam having transmitted through the PBS 11 proceeds to a quarter wavelength plate 12. The beam having proceeded to the quarter wavelength plate 12 is converted into a circular polarized beam in the quarter wavelength plate 12 and then proceeds to a compensator 13. The beam having proceeded to the compensator 13 is converted into a linear polarized beam in the compensator 13 and then proceeds to a ferroelectric liquid crystal (FLC) display device 14. The beam incident to the FLC display device 14 is converted into a circular polarized beam in the FLC display device 14 and then reflected from the FLC display device 14. The reflected beam is converted into a linear polarized beam in the FLC display device 14 and proceeds to the compensator 13. The beam having proceeded to the compensator 13 is converted into a circular polarized beam in the compensator 13 and then proceeds to the quarter wavelength plate 12. The beam incident to the quarter wavelength plate 12 is converted into a linear polarized beam in the quarter wavelength plate 12 and then proceeds to the PBS 11. In the beam proceeding from the quarter wavelength plate 12 to the PBS 11, a P polarized beam transmits through the PBS 11 and an S polarized beam reflects from the PBS 11 to proceed to a projection lens.
In FIG. 1, the compensator 13 is made of the same material as that of the FLC display device 14. The compensator 13 converts the polarized beam of the incident beam. The molecular axis of each pixel of the compensator 13 is aligned in the disorder state when a driving voltage is not applied thereto. The molecular axis of each pixel of the compensator 13 is aligned in any one direction of 0.degree. direction and +45.degree. direction based on a vertical axis according to an applied driving voltage. The compensator 13 looks as if the whole compensator 13 is made of a single pixel when a molecular axis of each pixel is aligned in the same direction. An angle where a molecular axis is aligned to be 0.degree. or +45.degree. is called a switching angle (referring to FIG. 2A).
In FIG. 1, the FLC display device 14 is a reflective liquid crystal display device and converts a polarized beam of the incident beam. The molecular axis of each pixel of the FLC display device 14 is aligned in the disorder state when a driving voltage is not applied thereto. The molecular axis of each pixel of the FLC display device 14 is aligned in any one direction of 0.degree. direction and -45.degree. direction based on a horizontal axis according to an applied driving voltage. An angle where a molecular axis is aligned to be 0.degree. or -45.degree. is called a switching angle (referring to FIG. 2B).
The compensator 13 and the FLC display device 14 of FIG. 1 are integrally formed.
FIG. 3 illustrates a table showing a switching angle with respect to a molecular axis of the compensator 13, a switching angle with respect to a molecular axis of the FLC display device 14, and transmissivity of a beam with respect to a projection lens, in the cases that the beam proceeding from the quarter wavelength plate 12 to the PBS 11 reflects from the PBS 11 and then proceeds to the projection lens (the white state), and transmits through the PBS 11 to then not proceed to the projection lens (the black state).
In FIG. 3, in the white state, each molecular axis of the compensator 13 and the FLC display device 14 is aligned to have a switching angle of 0.degree. and 0.degree. or +45.degree. and -45.degree.. Here, the transmissivity of the beam with respect to the projection lens is substantially 100%. Meanwhile, in the black state, each molecular axis of the compensator 13 and the FLC display device 14 is aligned to have a switching angle of 0.degree. and -45.degree. or +45.degree. and 0.degree.. Here, the transmissivity of the beam with respect to the projection lens has a small value.
However, since the beam transmitted from the quarter wavelength plate 12 transmits through the PBS 11 but does not proceed to the projection lens in the black state, the transmissivity of the beam with respect to the projection lens should be substantially 0%. However, as shown in FIG. 3, it can be seen that the transmissivity is not 0% in the black state.
Also, in order to maintain the transmissivity to be 0% in the black state, an angle of 90.degree. should be formed between the molecular axis of the compensator 13 and that of the FLC display device 14. However, as can be seen from FIG. 3, an angle of 135.degree. or 45.degree. is formed between the molecular axis of the compensator 13 and that of the FLC display device 14. Thus, the reflective display apparatus has a problem where a contrast ratio is lowered due to the luminance of a small amount of the beam transmitted through the projection lens in the black state.